UMTS system is a 3rd asynchronous mobile communication system using Wideband-Code Division Multiple Access (W-CDMA) based on Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS), which are mobile communication systems in Europe.
The 3rd Generation Partnership Project (GPP), which is standardizing the UMTS, is discussing a Long Term Evolution (LTE) system as a next generation mobile communication system of the UMTS system.
Aiming to commercialization in about 2010, the LTE realizes a high speed packet based communication at a data rate of 100 Mbps at maximum. To this end, diverse schemes are under consideration such as a method for reducing the number of nodes in a communication path by simplifying a network configuration and a method for making wireless protocols close to a radio channel.
FIG. 1 illustrates an evolved UMTS mobile communication system.
In FIG. 1, the Evolved UMTS Radio Access Network (E-UTRAN) 110 is simplified to a two-node structure including evolved Node Bs (eNBs) 120, 122, 124, 126 and 128, and anchor nodes 130 and 132.
A User Equipment (UE) or a terminal 101 accesses to an Internet Protocol (IP) network over the E-UTRAN 110.
The eNBs 120 through 128 correspond to Node Bs of the UMTS system and are connected to the UE 101 over radio channels. Unlike the Node B, the eNBs 120 through 128 conduct more complicated functions. In the LTE, every user traffic including real-time services such as Voice over IP (VoIP) using the IP is serviced in a shared channel. The eNBs 120 through 128 are responsible to aggregate and schedule status information of the UEs.
One eNB usually controls a plurality of cells. The eNB performs an Adaptive Modulation and Coding (AMC) which determines a modulation scheme and a channel coding rate based on the channel status of the terminal.
Similar to High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA) (or Enhanced Dedicated Channel (E-DCH) of the UMTS, Hybrid Automatic Repeat reQuest (HARQ) is performed between the eNB 120 through 128 and the UE 101. Since merely the HARQ cannot satisfy requirements of various Quality of Services (QoSs), an outer ARQ at an upper layer may be carried out between the terminal 101 and the eNB 120 through 128.
The HARQ raises the reception success rate by soft-combining the previously received data with the retransmitted data. The HARQ is used to increase the transmission efficiency in high-speed packet communications such as HSDPA and EDCH.
To realize the data rate of 100 Mbps maximum, it is expected that the LTE adopts Orthogonal Frequency Division Multiplexing (OFDM) in the bandwidth of 20 MHz.
FIG. 2 illustrates a home cell deployment.
In FIG. 2, it is assumed that a public cell 201 covers several to tens of kilometers and a home cell 203 covers merely several to tens of meters. The home cells may include a home cell 213 which allows only a particular UE 211 to access.
The public cell 201 determines a measurement period and a handover process by taking into account a time taken for the UE to move to another cell and a cell overlapping area.
When the UE moves from the home cell to the public cell, because of a small radius of the home cell, within a shorter time than required for the measurement and the handover, the normal handover is not carried out. As a result, the call may be disconnected over a certain time, data may be lost, or system fail may be caused.
To avoid those problems, it is necessary to shorten the measurement period and to simplify the handover process. Currently, since the home cell and the public cell use different Frequency Assignments (FAs), gaps for the measurement need to generate frequently.
However, since the data cannot be transmitted or received during the measurement, the performance is subject to degradation. In addition, when loose handover conditions lead to too many unnecessary handovers, resources are wasted and the performance is degraded.